After spending most of the summer away from home, on tour with his band, Dylan Chenfeld was thrilled to return to New York and slip into his old routines. But his first time back on an electric Citi Bike — his beloved “White Stallion,” as he calls it — he realized something was amiss. Pedaling into an intersection, he presumed the bike’s motor would propel him safely through the crossing traffic. But the boost never came, and before he knew it, he was skittering a few feet from the hood of a car.